A major problem which limits the usefulness and useful lifetime of materials exposed to ultraviolet (UV) radiation is degradation associated with such exposure. This degradation results from decomposition and other chemical reactions of the materials initiated by absorption of ultraviolet light. Among the undesired consequences of these chemical reactions can be discoloration of the material. Among the materials that are particularly affected by UV induced discoloration are solvents that comprise the media of functional devices such as electroactive devices, including electrochromic and electrophoretic devices.
Electrochromic devices are well known, e.g., U.S. Pat. Nos. 4,902,108 and 6,178,034. Such devices undergo a change in electromagnetic radiation transmission upon application of an electrical stimulus and have found use in a number of commercial applications. For example, they may be employed in glazings, e.g., energy efficient and privacy windows for architectural or automotive use, automotive rearview mirrors, displays, filters, eyewear, antidazzle and fog penetrating devices, and other applications where variable light transmission is desired. In many of these applications the device is routinely exposed to harmful environmental conditions, including exposure to UV light as from the sun.
Electrochromic devices are typically associated with a noticible change in color. Changes in other optical properties, such as in the degree of clarity and opacity and absorption in the IR, are also characteristics of such devices. The compositions of the present invention can also be employed as part of other electroactive devices associated with similar activity such as liquid crystal and suspended particle devices, including, as examples, electronic paper and privacy windows.
U.S. Pat. Nos. 6,614,578; 5,770,114; 6,207,083 and 6,045, 724, which are incorporated herein in their entirety by reference, disclose solvent containing media for electrochromic devices which protect the device by absorbing UV light either through an intrisic property of the solvent or by the addition of UV absorbing additives (UVAs), e.g., benzotriazoles, benzophenones, cyanoacrylates and others.
U.S. Pat. No. 5,148,305 claims compositions with certain cyanoacrylate UVAs in specified electrochromic soultions due to the greater solubility than similar solutions containing certain benzotriazoles, benzophenones, and/or oxanilides.
U.S. Pat. Nos. 6,614,578 and 5,770,114 disclose media for electrochromic devices containing benzotriazole UVAs that are substituted with groups to provide improved solubility.
U.S. Pat. No. 5,239,406, which is incorporated herein in its entirety by reference, discloses a multi-layered electrochromic glazing assembly, at least one layer of which comprises an additive for absorbing, blocking, and/or screening ultraviolet radiation. Said additive may be in one of the polymeric layers of the invention or in the electrochromic medium itself.
U.S. Pat. Nos. 6,143,209; 6,327,069; 6,404,532 and 6,545,793, which are incorporated herein in their entirety by reference, describe electrochromic devices comprising a solvent and optionally UV absorbers including benzophenones, cyanoacrylates, salicylates and benzotriazoles.
Even with the addition of UV absorbing additives, discoloration of the media upon exposure to UV light remains a problem. In addition to UVA's, other stabilizing additives, including hindered amine light stabilizers (HALS), have been used in electrochromic media, for example, U.S. Pat. Nos. 6,178,034 and 5,239,406, and U.S. Pat Appl Pub. 2002/0141032.
Electrophoretic devices are also well known, particularly those comprised of suspended particles, e.g., U.S. Pat. Nos. 4,772,103; 6,753,844 and 6,753,999 which are incorporated herein in their entirety by reference. They can be commercially employed in many of the same applications as electrochromic devises, e.g., glazings and displays and are subject to similar photodegradation. U.S. Pat. No. 5,467,217, incorporated herein in its entirety by reference, discloses electrophoretic light valve compositions stabilized with UV light absorbers.
Several classes of hindered amine light stabilizers (HALS) are known in the art. These classes are differentiated by the substitution at the hindered N-atom, for example, hydrogen, alkyl, acyl, alkoxy, hydroxyalkoxy, acyloxy. Polymeric deirivatives of HALS are also known.
While the use of UVAs and HALS together is well known in the art of UV stabilization, e.g., U.S. Pat. No. 4,314,933, little guidance exists as to which HALS provide the greatest utility in preventing discoloration of the solvent comprising the media of electroactive devices such as electrochromic or electrophoteric devices.
U.S. Pat Appl Pub no. 2002/0141032, specifically mentions the use of a N-alkoxy HALS. U.S. Pat. No. 6,178,034 generically discloses the use of mixtures of additives, including, among others, UVAs and HALS, in the electrolyte layer of an electrochromic device.
We have found that certain HALS, substituted on the hindered nitrogen by hydrogen or alkyl are much more effective than others in preventing the yellowing of solutions containing UVAs found in electroactive devices. The presence os these HALS also extends the usable lifetime of the UVAs.